Manufacture of extrusion dies

ABSTRACT

An extrusion die (11) comprises a die cavity (12) having a shape corresponding to the cross-sectional shape of the required extrusion, and a preform chamber (19) in communication with the die cavity (12), the preform chamber (19) being of generally similar shape to the die cavity (12) but of greater cross-sectional area, so that regions of the preform chamber (19) communicate with corresponding regions respectively of the die cavity (12). Each region of the preform chamber (19) has a bearing length (20) which is so determined in relation to its dimensions and position that, in use, extrusion material passing through each region of the preform chamber (19) is constrained to move at a velocity such that the material passes through all regions of the die cavity (12) at a substantially uniform velocity. The die cavity (12) itself is of uniform, preferably zero, bearing length so that the extrusion process is controlled solely by adjustment of the preform chamber (19), such adjustment then being readily quantifiable and repeatable.

This application is a 371 of PCT/GB96/01595 filed Jul. 4, 1996.

The invention relates to extrusion dies used for producing elongateprofiles in metal (such as aluminium) plastics etc. In an extrusionprocess it is necessary for all parts of the material being extruded topass through the die at substantially the same velocity, since if thisis not the case the extruded profile is likely to be deformed.

As is well known, in an extrusion die the velocity of the extrusionmaterial through the die, at any particular region of the die cavity,depends on the width of the die cavity in that region, its positionrelative to the centre of the die, and the bearing length of the diecavity (i.e. its length in the extrusion direction) in that region.

Since the width and position of each region of the die cavity areessentially determined for any particular profile to be extruded, it isnormally necessary to control the velocity by adjusting the bearinglength of the die cavity in different regions thereof so that thevelocity of extrusion material is as uniform as possible through thewhole area of the die cavity. Thus, a narrow part of the die cavity willrequire a shorter bearing length than a wider part of the cavity inorder to achieve the same velocity.

This required variation in bearing length (known as the bearing contour)is normally achieved by forming in the back face of the die, i.e. theface furthest from the billet of material to be extruded through thedie, an exit cavity which corresponds to the general shape of the diecavity plus an all-round clearance. The depth of the exit cavity is thenvaried so as to adjust the effective bearing length of the die cavityitself.

Various methods of this kind for manufacturing an extrusion die aredescribed, for example, in British Patent Specifications Nos. 2143445and 2184371.

There are numerous well known methods and techniques for providing therequired correlation between bearing length and die cavity shape andposition in order to achieve uniform flow. For example, the requiredbearing lengths may be achieved by trial-and-error methods based on theknowledge of an experienced die designer or, increasingly, computerprograms are available to calculate required bearing lengths from theshape and position of the die cavity.

However, the extrusion dies resulting from such prior art methods maysuffer from certain disadvantages. For example, the surface of theextruded profile may be longitudinally marked by a part of the diecavity where there are two adjoining regions of significantly differentbearing lengths, as may frequently occur. Furthermore, since the diecavity itself has to be worked on and adjusted to control the flow ofextrusion material, it may not be possible to form the die from amaterial which cannot be readily worked, or to provide it with a surfacefinish, such as nitriding, which might otherwise be desirable to give abetter finish to the profile. It would therefore be desirable to achievesubstantially uniform flow through a die cavity which has asubstantially uniform, fixed bearing length so as to avoid marking ofthe profile due to changes in bearing lengths and to allow the die to beformed from a material, and have a surface finish, to give the bestpossible strength and wear resistance as well as to provide the finestpossible finish on the extruded profile.

One method of achieving such an effect is described in European PatentSpecification No. 0569315. In the method described in thatspecification, there is provided on the front, or entry, side of the diecavity an enlarged entry cavity the sides of which converge as theyextend towards the cavity in the extrusion direction so as to provide an"entry angle". This "entry angle" is calculated in reciprocal ratio withthe width of each region of the die cavity. Selection of different entryangles to different regions of the die cavity thus controls the velocityof extrusion material towards the die cavity in such manner that, at theentry to the die cavity, the velocity of the extrusion material at eachregion is such as to result in a substantially uniform velocity throughthe whole area of the die cavity. Accordingly, the die cavity itself maybe of substantially constant bearing length. In a preferred embodimentthe entry angle is provided by forming the entry cavity with a series ofsteps extending inwardly towards the die cavity. The steps are ofconstant depth and the entry angle is adjusted by varying the width ofthe steps.

While such arrangement has met with some success, it may suffer fromcertain disadvantages. For example, where the die cavity is formed withsections which are closely spaced from one another there may beinsufficient room on the entry side of each section to provide separateand individual entry angles for each region, since the adjacent steppedentry cavities would overlap. Consequently, in practice such closelyadjacent sections of the die cavity have to communicate with a singlestepped entry cavity. This means that there is no individual controlover flow through these adjacent regions of the die cavity and this mayresult in non-uniform flow through the regions if they are of differentwidths. Furthermore, the adjustment of the flow rate by adjustment ofthe entry angle does not make use of the long established and well knowntechniques for controlling velocity by adjusting bearing length, withthe result that die designers must learn entirely new, and unfamiliar,techniques and parameters in order to put the system into operation.

Also, although the "entry angle" may be calculated for each region ofthe die cavity, it is in practice also necessary to make minoradjustments in order to correct variations in velocity which may show upin initial testing of the die. Such minor adjustments may be effected byadjusting the bearing length of the die cavity in a particular region,but this loses the advantage of having a die cavity of substantiallyconstant bearing length. However, it may be difficult to make accurateminor adjustments to the entry angle which is the only other means forvarying the velocity through a region of the die. This is presumably whythe stepped arrangement is preferred since it may be easier to adjustthe width of a series of steps than it is to accurately adjust the angleof a continuous inclined surface. However, the provision of the stepsmay provide considerable resistance to the flow of material into the diecavity with the result that the overall velocity of the extrusionmaterial through the die is reduced. This is undesirable since theproductivity of an extrusion installation depends on the speed withwhich extrusions are produced. Also, the stepped arrangement may causethe generation of excessive heat.

It is also known to provide a lead-in plate on the front side of thedie, provided with apertures which communicate with the die cavities.However, such lead-in plates are generally of constant thickness and thevelocity of extrusion material passing through the apertures in thelead-in plate may only be adjusted by adjusting the width of suchapertures. This is not sufficiently precise to provide accurate velocitycontrol, and conventional correction of the die cavity itself is alsorequired. For continuous extrusion it is also common practice to providea weld plate on the front side of the die. In this case the trailing endof each metal billet is sheared off at the front surface of the weldplate and is engaged by the leading surface of a new billet whichbecomes welded to the end of the previous billet as the junction betweenthe two billets passes through the weld plate. However, again, the weldplate is not used to control the flow of metal precisely, and correctionof the die cavity itself is still required.

The present invention sets out to provide improved forms of extrusiondie, and methods of manufacture of such dies, which may overcome many orall of the above-mentioned disadvantages of the prior art systems and ina preferred embodiment, provides a fully controlled system where nocorrection of the die cavity itself is required.

According to the invention there is provided an extrusion die comprisinga die cavity having a shape corresponding to the cross-sectional shapeof the required extrusion, and a preform chamber in communication withthe die cavity, the preform chamber being of generally similar shape tothe die cavity but of greater cross-sectional area, so that regions ofthe preform chamber communicate with corresponding regions respectivelyof the die cavity, each region of the preform chamber having a bearinglength which is related to the dimensions and position of said region sothat, in use, extrusion material passing through each region of thepreform chamber is constrained to move at a velocity such that thematerial passes through all regions of the die cavity at a substantiallyuniform velocity.

Since the velocity of the extrusion material is fully controlled in thepreform chamber, i.e. before it reaches the die cavity, the die cavityitself may be of constant bearing length in all regions thereof, withthe advantages referred to above. The velocity of metal through thepreform chamber is adjusted by adjusting the width and bearing length ofthe preform chamber. This enables the wealth of experience and/orcomputer programs already used in the designing of conventional diecavities to be employed, resulting in accurate control of the velocity.Furthermore, since no "entry angle" is required, the side walls of thepreform chamber may be parallel or substantially parallel, so that themaximum width of the preform chamber may be significantly less than themaximum width of the entry cavity in the prior art "entry angle"arrangement referred to above, with the result that there is room toprovide a separate region of the preform chamber for each region of thedie cavity. If two regions of the die cavity are particularly closelyspaced, the enlarged preform chamber communicating with each region maybe made correspondingly narrow, the velocity being controlled byreducing the bearing length of the preform chamber. Alternatively, ifthe shape of the die cavity permits this, the regions of the preformchamber may be offset relative to their corresponding regions of the diecavity so that they do not interfere with one another, while remainingin communication with their corresponding regions of the die cavity.

To provide precise control of the flow through the preform chamber, theside walls of the chamber are preferably exactly parallel.

By appropriate selection of the width of the different regions of thepreform chamber, the number of regions of the preform chamber requiringa different bearing length may be reduced. This allows the number ofvariable parameters for controlling the flow of metal through the dieaperture to be reduced thus simplifying correction of the die andrendering such correction more repeatable and reliable.

As mentioned above, variations in velocity can cause the extrudedprofile to be deformed and varying the bearing length within the diecavity itself can lead to surface marking of the profile. The presentinvention may therefore achieve the production of high quality profiles.Equally importantly however, the invention enables the manufacturingprocess itself to be controlled and improved. For example, an extrusiondie will normally incorporate a number of similar die cavities spacedapart over the face of the die, so as to produce several extrudedprofiles simultaneously. As they are extruded, the profiles are drawn bya single puller device. Accordingly, it is necessary for the profilesfrom all of the die cavities to be extruded at the same speed sinceotherwise the puller device may stretch and thus deform any of theprofiles which are being extruded at a slightly slower speed than therest. Since the present invention allows the speeds of extrusion to becontrolled very accurately it becomes possible to unify the speeds ofextrusion from the various die cavities in the die. The invention alsoallows the overall velocity of extrusion to be increased, as will bedescribed, thus allowing the productivity of the die to be increased ina reliable and controlled manner.

Since the velocity through each region of the die cavity is controlledin the preform chamber before the die cavity is reached, the die cavitywill produce an extruded profile which is of exactly the same shape asthe die cavity and it is not necessary, as has hitherto been the case,to build deformations into the die cavity in order to correct theprofile of the extrusion emerging from it. For example, withconventional methods it is frequently necessary, for some shapes ofprofile, to incline the walls of the bearing portion of the die cavityin one direction or another in order to compensate for some deficiencyin the shape of the profile which becomes apparent in testing. Also, forexample, where two portions of a profile are required to be at aspecified angle to one another, it may be necessary for thecorresponding portions of the die cavity to be at a slightly differentangle in order to achieve the required angle in the extruded profile.Some of these adjustments in the shape of the die aperture may be veryslight and may be lost or diminished if the die is not carefully andproperly maintained over a prolonged period of use. Thus, cleaning andpolishing of the die aperture can, over time, remove slight correctionalvariations in the shape of the die aperture so that although the dieproduces the correct profile when new, it changes with use to begin toproduce a slightly deformed profile. This problem does not arise withthe present invention where the control of the metal flow is effectedbefore the metal reaches the die aperture. This sort of deliberatedeformation of the die cavity can be avoided with the present inventionwhere the extrusion material is fully controlled in the preform chamberbefore it reaches the die cavity and may be so controlled that theextruded profile produced by the die cavity is exactly in accordancewith the shape of the die cavity itself.

The alterations and corrections which a conventional die corrector maymake to a die, in order to achieve the desired profile, may be slightand subtle, being based on the die corrector's long experience and oftenbeing intuitive. Such corrections may therefore be difficult orimpossible to record and to repeat reliably over a succession of similardies. By contrast, in the present invention the desired profile isachieved by adjusting a few clearly-defined parameters of the preformchamber. These parameters may be measured and recorded, for example in acomputer program, and repeated continually, by precise machine methods,in a succession of dies to give entirely consistent results.Conventional die correction may require much hand work, which isinherently difficult to repeat precisely. The present invention mayallow all shaping of the preform chamber and die cavity to be carriedout by machine, so as to be inherently repeatable.

As mentioned above, the die cavity may be of substantially constantbearing length in all regions thereof. In particular, the inventionallows all regions of the die cavity to be of substantially zero bearinglength.

It is known to provide extrusion dies of zero bearing length, and forexample such dies are described in European Patent Specification No.0186340. However, as acknowledged in that specification, the design of aconventional zero bearing length die is such that modification of theprofile of the aperture to hasten or slow the passage of metal is notpossible. Accordingly, zero bearing length dies have hitherto beenregarded as mainly suitable for extruding the minority of sections whoseconfiguration does not require adjustment or correction. If aconventional zero bearing length die does not produce an extrusion ofthe required profile, there is no way in which the die can be corrected.However, since the present invention allows control of the velocity ofthe metal upstream of the die, it allows the use of zero bearing lengthdies for virtually all types of section. Thus, the present inventionallows the advantages of zero bearing length dies to be combined withreliable correction and control.

A die cavity of substantially zero bearing length may be formed byproviding in the die plate a die aperture which is negatively taperedthroughout its length, i.e. the walls of the die aperture diverge asthey extend from the front surface to the back surface of the die plate.As mentioned in EP 0186340 a negative taper angle of at least 0.8°0 ispreferred so that any friction stress between the walls of the die andmetal flowing through it is negligible. It is believed that a negativetaper angle of about 1.5° is more reliable.

It will be appreciated that it is in practice impossible to provide adie cavity which is literally of zero bearing length, since there willnormally be a small radius at the junction between the negativelytapered die cavity and the front surface of the die plate. EP 0186340relates to arrangements where this radius of curvature is not greaterthan 0.2 mm. However, for the purposes of this specification the diecavity is regarded as having zero bearing length where the die cavityincreases in width as it extends away from the front face of the dieplate, regardless of the radius of curvature at the upstream end of thedie cavity.

In any of the arrangements according to the invention the region of thepreform chamber which is of minimum bearing length may also be ofsubstantially zero bearing length, increasing to a maximum the overallvelocity of extrusion.

At least some of said regions of the preform chamber may each have awidth which is the same predetermined percentage greater than the widthof the respective corresponding region of the die cavity. Alternativelyor additionally, at least some of said regions of the preform chambermay each have a width which is greater than the width of the respectivecorresponding region of the die cavity by the same predetermined amount.

The width of said regions of the preform chamber are preferablysubstantially symmetrically disposed in relation to the width of thecorresponding region of the die cavity. However, as previouslymentioned, the width of one or more of said regions of the preformchamber may be offset in relation to the width of the correspondingregion of the die cavity.

Preferably the bearing length of each region of the preform chamber isprovided by a bearing part thereof which is immediately adjacent thecorresponding region of the die cavity.

Each region of the preform chamber may include a part which is upstreamof the bearing part which provides the bearing length, and whichincreases in width as it extends away from said bearing part.

The die cavity and preform chamber are preferably formed in separatecomponents which are clamped together with the preform chamber incommunication with the die cavity. Alternatively the die cavity andpreform chamber may be integrally formed in a single component. However,an advantage of forming the preform chamber and die cavity in separatecomponents is that it may allow the preform chamber component to bere-used with a new die cavity component should the original die cavitycomponent wear out.

The invention also includes within its scope a method of manufacturingan extrusion die comprising forming the die with a die cavity having ashape corresponding to the cross-sectional shape of the requiredextrusion, and a preform chamber in communication with the die cavity,the preform chamber being of generally similar shape to the die cavitybut of greater cross-sectional area, so that regions of the preformchamber communicate with corresponding regions respectively of the diecavity, and adjusting the bearing lengths of different regions of thepreform chamber in relation to the dimensions and position of thoseregions so that, in use, extrusion material passing through each regionof the preform chamber is constrained to move at a velocity such thatthe material passes through all regions of the die cavity at asubstantially uniform velocity.

The following is a more detailed description of embodiments of theinvention, by way of example, reference being made to the accompanyingdrawings in which:

FIG. 1 is a diagrammatic front face view of an extrusion die formed withtwo simple cavities,

FIG. 2 is a diagrammatic section on the Line 2--2 of FIG. 1,

FIG. 3 is a diagrammatic section on the Line 3--3 of FIG. 1,

FIG. 4 is a front face view of an extrusion die showing two die cavitiesof slightly more complex form than FIG. 1,

FIG. 5 is a section on the Line 5--5 of FIG. 1,

FIG. 6 is a diagrammatic front face view of part of a further form ofdie cavity,

FIG. 7 is a diagrammatic section on the line 7--7 of FIG. 6,

FIG. 8 is a diagrammatic section through a die having a die cavity ofzero bearing length,

FIG. 9 is a diagrammatic section through another form of die,

FIG. 10 is a diagrammatic section through a further form of die,

FIG. 11 is a similar view of a modified version of the cavity of FIG.10, and

FIG. 12 is a diagrammatic section through a die cavity incorporatingcooling.

FIG. 1 shows the front face 10 of an extrusion die 11 formed with twocavities 12 and 13 of generally flattened Z-shape.

In a conventional prior art construction each die cavity 12 or 13 wouldcommunicate with an enlarged divergent exit cavity formed in the backface of the die plate. The bearing length of different regions of thedie cavity, i.e. its dimension in the direction of extrusion, would beadjusted by adjusting the depth of this exit cavity. By this means thebearing length of each part of the die cavity would be adjusted in amanner to result in a substantially uniform velocity of the extrusionmaterial through all parts of the die cavity.

By contrast, in accordance with the present invention, the front face ofthe die is formed with a preform chamber through which the extrusionmaterial is forced before it reaches the die cavity 12 or 13, thusenabling the velocity of the extrusion material to be adjusted before itreaches the die cavity itself.

Referring to FIG. 2 it will be seen that the die 11 comprises a backplate 14 in which the die cavity 12 itself is formed. All parts of thedie cavity 12 have a constant bearing length 15 which may, for example,be 2 mm. An exit cavity 16 leads from the die cavity 12, the walls ofthe cavity diverging as they extend to the back face 17 of the die plate14.

Clamped rigidly to the back plate 14 is a front plate 18 which is formedwith a preform chamber 19. The preform chamber is generally similar inshape to the die cavity 12 but the width of all regions of the preformchamber is greater than the width of the corresponding regions of thedie cavity 12. As may be seen from FIG. 1, in the case of the upper diecavity 12 the preform chamber 19 has a width which is increased by 50%all around the die cavity 12 so that the overall width of each region ofthe preform chamber 19 is twice the overall width of the correspondingregion of the die cavity. Such arrangement will be referred to as a "50%growth" arrangement.

In accordance with the present invention the bearing length 20 (see FIG.2) of each region of the preform chamber 19 is calculated in accordancewith the width of the preform chamber in that region, and in accordancewith its distance from the centreline 21 of the die, to give a requiredvelocity of extrusion material as it enters the die cavity itself. Thevelocity at entry to each region of the die cavity is selected such thatthe rate of subsequent flow through all regions of the die cavity issubstantially uniform. The bearing length 20 of the preform chamber iscontrolled by milling into the front face 10 of the front plate 18 anentry cavity 22 of appropriate depth to give the required resultantbearing length 20 to the preform chamber 19.

The entry cavity 22 comprises a flat narrow shoulder 22a, to define theinlet end of the preform chamber 19 exactly, and surfaces 22b inclinedat approxirnately 45° away from the chamber 19. Such inclination isnecessary to ensure that these surfaces do not act as a bearing on theextrusion metal so as to alter the bearing effect of the preform chamber19.

The use of a preform chamber 19 where the side walls of the preformchamber are parallel enables the velocity to be controlled, by adjustingthe bearing length 20, using well established means of calculating therequired bearing length to achieve the required velocity. Also, sinceadjustments to the die to adjust the velocity do not require anyalteration to the die cavity 12 itself, as is the case in most prior artmethods, the die cavity 12 may be formed in any material to give therequired strength and wear resistance without taking into account anynecessity of being able to adjust the bearing length of the die cavityafter it has been initially formed. Also, since the bearing cavityitself remains unchanged, it may be coated with an appropriate finish,such as by nitriding, so as to give the best possible surface finish tothe extruded profile.

Also, since the die cavity 12 itself is of constant bearing length, thisalso inherently results in a finer finish on the extruded profile, incontrast to the prior art arrangements where the extrusion is likely tobe marked where it passes through a region of the die cavity where twodifferent bearing lengths are adjacent one another.

The extent of increase in width, or "growth", of the preform chamber inrelation to the die cavity may be of any required value, depending onthe size and shape of the die cavity itself and its position in relationto the centreline of the die. By way of example, FIG. 1 also shows a diecavity 13 where the preform chamber 23 exhibits 200% growth, i.e. theincreased width of the preform chamber on each side of the die cavity istwice the width of the die cavity 13 itself. Again, an entry cavity 24is milled into the front face 10 of the front plate 18 of the die, thedepth of the entry cavity 24 being selected to give a required bearinglength to the preform chamber 23 and hence a required velocity of theextrusion material as it reaches the die cavity 13 itself.

In the case, such as those shown in FIG. 1, where the percentage"growth" of the preform chamber is constant for all regions of the diecavity, the velocity of extrusion material through the preform chamberis controlled solely by adjusting the bearing length of the preformchamber leading to each region. However, in some cases, with morecomplex profiles, it may be advantageous also to vary the percentagegrowth of the preform chamber in different regions of the die cavity,and FIGS. 4 and 5 show an example of this.

Referring to FIGS. 4 and 5, the extrusion die 25 again comprises a frontplate 26 and a back plate 27. The back plate 27 is formed with twoidentical die cavities, an upper cavity 28 and a lower cavity 29. Eachdie cavity has a uniform bearing length of, for example 2 mm, in allregions thereof and leads to an exit cavity 30 which diverges outwardlyto the back face 31 of the die.

The front plate 26 is formed with preform chambers 27 and 33 whichcommunicate with the die cavities 28 and 29 respectively and entrycavities 32 and 34 are milled in the front plate 26 to communicate withthe die preform chambers respectively.

As best seen in FIG. 4, the two die cavities 28 and 29 are of the sameshape, the upper cavity 28 comprising a central region 28a of generallyflattened Z-shape, an end region 28b of greater width than the centralregion 28a, and an opposite end region 28c of smaller width than thecentral region. For example, the central region may have a width of 2mm, the end region 28b a width of 4 mm, and the end region 28c a widthof 1 mm.

As in the previous arrangement the preform chamber 27 is of generallysimilar shape to the die cavity 28, and has 50% growth, i.e. the widthof the preform chamber, on each side of the die cavity, is increased by50% of the width of the die cavity.

Also as in the previous arrangement, the bearing lengths of thedifferent regions of the preform chamber 27 are adjusted in relation tothe width and position of the regions of the preform chamber, and henceof the regions of the die cavity with which they communicate. Thus, theenlarged region 27b of the preform chamber will require a significantlygreater bearing length than the region 27a, as may be seen from FIG. 5,in order to reduce the velocity to what is appropriate for the largerarea of the region of the die cavity, whereas the smaller region 27c ofthe preform chamber will require a smaller bearing length than theregion 27a.

In some cases finer control of the velocity of the extrusion materialmay be achieved by also varying the percentage growth of differentregions of the preform chamber, in addition to varying their bearinglengths, and such an arrangement is shown in the case of the lower diecavity 29 in FIG. 4. In this case the central region 33a of the preformchamber 33 still has 50% growth, but the enlarged end region 33b of thepreform chamber has only 25% growth. The opposite end region 33c of thepreform chamber, communicating with the reduced end region 29c of thedie cavity, has 200% growth.

Looked at another way, the regions 33a and 33b of the preform chambermay be regarded as having a width which is greater than the width of therespective corresponding regions 29a and 29b of the die cavity by thesame predetermined amount, even though the region 29b of the die cavityis wider than the region 29a.

The effect of the proportionally reduced growth of the preform chamberregion 33b is to decrease the velocity of the extrusion material throughthat region of the preform chamber compared with the velocity throughthe region 33a, so that a shorter bearing length is required in region33b to achieve the required velocity through the region 29b of the diecavity. Similarly the increase in width of the region 33c of the preformchamber serves to increase the velocity of the extrusion material in amanner appropriate for such a narrow region of the die cavity. Thisovercomes the possible problem that, with a uniform percentage growth,it may not be possible, by adjustment of the bearing length alone, toachieve sufficient velocity of the extrusion material in the preformchamber 33c to ensure that the material passes at the required velocitythrough the region 29c of the die cavity.

In all of the above arrangements according to the invention theprovision of a preform chamber corresponding in shape to the die cavitythus provides great flexibility in control over the velocity of theextrusion material through the die to enable the optimum extrusionconditions to be obtained.

It will be appreciated that the simple shapes of die cavity shown aremerely by way of example and the invention is applicable to any profileshape. For example, the invention is applicable to extrusion dies forextruding hollow shapes. In this case each preform chamber will beformed partly in the male portion of the die and partly in the femaleportion so as to provide a preform chamber communicating with the wholeof the die cavity.

In the arrangements of FIGS. 1-5 each region of the preform chamber issubstantially symmetrical with respect to the corresponding region ofthe die cavity, that is to say the preform chamber region overlaps thedie cavity region by a similar amount on each side. However, this is notessential and in some configurations of die cavity certain regions ofthe cavity may be so close together that symmetrically disposed regionsof the preform chamber would overlap. In such circumstances the regionsof the preform chamber may be offset with respect to the correspondingregions of the die cavity so that they do not overlap and may thereforehave separate effects on their respective regions of the die cavity.Such an arrangement is shown in FIGS. 6 and 7.

As best seen in FIG. 6, the die cavity 35 is formed at one end toprovide two spaced parallel limbs 36. The limbs 36 of the die cavity maybe so close that if the corresponding regions 37 of the preform chamberwere symmetrically disposed with respect to the regions 36 of the diecavity, they would overlap, thus interfering with the correctcontrolling effect of the preform chamber. Accordingly, in this case theregions 37 of the preform chamber are offset with respect to theircorresponding regions 36 of the die cavity, so as to form two separateand distinct regions. Each region 37 of the preform chamber thereforecan be adjusted to control accurately the flow of metal to itscorresponding region of the die cavity. The offsetting of the regions ofthe preform chamber has no significant adverse effect on the operationof the invention. Provided that the preform chambers result in theextrusion metal reaching the die cavity at uniform velocity, it does notmatter where the preform chambers are located in relation to the diecavity.

Since the velocity of the extrusion material through a region of the dieis increased by reducing the bearing length in that region, the overallvelocity of the material through the die may be increased by reducingall bearing lengths. In the majority of conventional extrusion dies itis necessary to retain significant bearing lengths in all regions of thedie cavity itself, since differential variation in such bearing lengthsis the only way of controlling velocity through the different regions ofthe die cavity. The present invention, however, allows the use of a diecavity of uniform bearing length. Accordingly, the present invention maybe used with a die cavity of so-called zero bearing length, aspreviously discussed, and one such arrangement is shown in section inFIG. 8.

In this arrangement the die plate 38 is formed with a die cavity 39having an inlet aperture 40 in the shape of the required extrusion. Thewalls 41 of the die cavity are negatively tapered, for example at 1.5°,i.e. they diverge slightly as they extend away from the aperture 40. Thedie plate is cut away at the downstream end of the die cavity 39, inconventional manner, as indicated at 42.

Since the walls 41 are negatively tapered they do not apply anysignificant frictional restraint to metal passing through the aperture40 and the metal is shaped solely by the corners 43 around the aperture40 so that the bearing length of the die cavity is essentially zero. Itwill be appreciated, however, that the corners 43 require to be smoothso as to provide a good surface finish on the extruded profile. Thesecorners will therefore be slightly radiused so that, in practice, therewill be a bearing length which is so small as to be negligible, ratherthan an actual zero bearing length.

As in all embodiments of the present invention, the velocity ofextrusion material through the aperture 40 is controlled by the bearinglength of the different regions of the enlarged preform chamber on theupstream side of the die cavity. As previously described, the regions ofthe preform chamber upstream of the control bearing length 44a aretapered outwardly, as indicated at 45 in FIG. 8, so that there isinsignificant risk of such parts of the preform chamber plate 44 havingany bearing effect on the extrusion material passing through it.

Another way of increasing the overall velocity of material through thedie is to reduce as far as possible the bearing lengths of the differentregions of the preform chamber.

In all the arrangements previously described, the bearing length portionof each preform chamber region is preferably as close as possible to thedie cavity. However, the invention does not exclude arrangements wherethe bearing lengths of the preform chamber regions are spaced upstreamfrom the corresponding regions of the die cavity. FIG. 9 shows anarrangement where the preform chamber region 50 has a zero bearinglength aperture 51 spaced upstream of a zero bearing length die cavity52. This arrangement minimises the overall bearing length of the die andthus provides for maximum velocity of extrusion material through thedie.

In order to retain control of velocity through all regions of the die,only the region of the preform chamber requiring minimum bearing lengthwill be of zero bearing length. However, this will enable the bearinglengths of the other regions to be reduced by a corresponding amount, aswill be described with reference to FIGS. 10 and 11.

FIG. 10 shows an arrangement in accordance with the present inventionwhere regions 46, 47 and 48 of the preform chamber are of differentbearing lengths, region 46 being of the shortest bearing length.However, the same effect may be achieved by reducing the bearing lengthof all regions of the preform chamber by an amount equal to the bearinglength of the smallest region 46. As shown in FIG. 11, this may beeffected by reducing the bearing length of the preform chamber 46 tozero by applying a negative taper to the sides of the chamber asindicated at 46a. The bearing lengths of the other preform chambers arereduced by a corresponding amount by negatively tapering a similarlength portion thereof, as indicated at 47a and 48a. Since the bearinglengths of the three regions of the preform chamber have the samerelationship, the velocity of the extrusion material as it reaches thedie plate 49 is uniform. However, the overall velocity of the materialis increased as a result of the reduction in effective bearing length ofall regions 46, 47 and 48 of the preform chamber.

In the arrangements described above the die comprises a separate dieplate and preform chamber plate, the two plates being clamped togetherface-to-face. However, in some circumstances it may be desirable andpossible to combine the two plates into a single integral plate formedwith the appropriate apertures. However, the two-plate arrangement willusually be preferred since it facilitates correction of the bearinglengths in the preform chamber plate and also allows the preform chamberplate to be re-used if the die plate wears out first, which is likely tobe the case.

FIG. 12 shows another situation where a two-plate arrangement is to bepreferred.

In some circumstances it may be desirable to cool the die and theextrusion material as it passes through the die cavity to reduce therisk of local melting. Cooling of the extrusion material is usually doneby injecting a cooled inert gas, usually nitrogen, into the downstreamregion of the die plate, but cooling of the die itself may be difficult.Two-plate arrangements according to the present invention enable suchcooling to be effected in a simple and convenient way, as illustrateddiagrammatically in FIG. 12. In this case a main channel 53 is formed inthe die plate 54 closely adjacent the die cavity 55 and passages 56extend laterally from the channel 53 to open into the downstream portionof the die cavity. The preform chamber plate 57 then closes the channel53. Cooled nitrogen is then pumped under pressure into the channel 53,thereby cooling the die itself, and is fed therefrom along the passages56 to cool the extrusion material passing through the die cavity.

What is claimed is:
 1. An extrusion die comprising a die cavity having ashape corresponding to the cross-sectional shape of the requiredextrusion, and a preform chamber in communication with the die cavity,the preform chamber being of similar shape to the die cavity but ofgreater cross-sectional area, so that regions of the preform chambercommunicate with corresponding regions respectively of the die cavityand, in use, extrusion material passing through all regions of the diecavity are constrained to move at a substantially uniform velocity, thedie cavity including a number of regions of constant bearing length, thepreform chamber having different bearing lengths in different regionsthereof, and each region of the preform chamber which corresponds to oneof said regions of constant bearing length having a bearing length whichis related to the dimensions and position of said region of the preformchamber so that, in use, extrusion material passing through each saidregion of the preform chamber is constrained to move at a velocity suchthat the material subsequently passes at a uniform velocity through eachof said corresponding regions of the die cavity which are of constantbearing length, wherein the bearing length of each region of the preformchamber is provided by a bearing part thereof which is immediatelyadjacent the corresponding region of the die cavity.
 2. An extrusion dieaccording to claim 1, wherein all regions of the die cavity are ofconstant bearing length.
 3. An extrusion die according to claim 1,wherein said regions of the die cavity which are of constant bearinglength are of zero bearing length.
 4. An extrusion die according toclaim 1, wherein a region of the preform chamber which is of minimumbearing length is of zero bearing length.
 5. An extrusion die accordingto claim 1, wherein at least some of said regions of the preform chambereach have a width which is a predetermined percentage greater that thewidth of the respective correspoding region of the die cavity.
 6. Anextrusion die according to claim 1, wherein at least some of saidregions of the preform chamber each have a width which is greater thanthe width of the respective corresponding region of the die cavity bythe same predetermined amount.
 7. An extrusion die according to claim 1,wherein the width of at least one of said regions of the preform chamberis symmetrically disposed in relation to the width of the correspondingregion of the die cavity.
 8. An extrusion die according to claim 1,wherein the width of at least one of said regions of the preform chamberis offset in relation to the width of the corresponding region of thedie cavity.
 9. An extrusion die according to claim 1, wherein eachregion of the preform chamber includes a part which is upstream of abearing part which provides the bearing length, and which upstream partincreases in width as it extends away from said bearing part.
 10. Anextrusion die according to claim 9, wherein a shoulder is provided atthe junction between said bearing part and said upstream part of thepreform chamber.
 11. An extrusion die according to claim 1, wherein thedie cavity and preform chamber are formed in separate components whichare clamped together with the preform chamber is communication with thedie cavity.
 12. An extrusion die according to claim 1, wherein the diecavity and preform chamber are integrally formed in a single component.13. A method of manufacturing an extrusion die comprising forming thedie with a die cavity having a shape corresponding to thecross-sectional shape of the required extrusion, and a preform chamberin communication with the die cavity, the preform chamber being ofsimilar shape to the die cavity but of greater cross-sectional area, sothat the regions of the preform chamber communicate with thecorresponding regions respectively of the die cavity, each region of thepreform chamber having a bearing part which is located immediatelyadjacent the corresponding region of the die cavity, adjusting thebearing length of the bearing part of different regions of the preformchamber in relation to the dimensions and position of those regions,without altering the bearing lengths of the corresponding regions of thedie cavity, so that, in use, extrusion material passing through eachregion of the preform chamber is constrained to move at a velocity suchthat the material passes through all regions of the die cavity at auniform velocity.
 14. An extrusion die comprising a die cavity having ashape corresponding to the cross-sectional shape of the requiredextrusion, and a preform chamber in communication with the die cavity,the preform chamber being of similar shape to the die cavity but ofgreater cross-sectional area, so that regions of the preform chambercommunicate with corresponding regions respectively of the die cavityand, in use, extrusion material passing through all regions of the diecavity are constrained to move at a substantially uniform velocity, thedie cavity including a number of regions of constant bearing length, thepreform chamber having different bearing lengths in different regionsthereof, and each region of the preform chamber which corresponds to oneof said regions of constant bearing length having a bearing length whichis related to the dimensions and position of said region of the preformchamber so that, in use, extrusion material passing through each saidregion of the preform chamber is constrained to move at a velocity suchthat the material subsequently passes at a uniform velocity through eachof said corresponding regions of the die cavity which are of constantbearing length, wherein the bearing length of each region of the preformchamber is provided by a bearing part, the bearing parts being locatedupstream of the die cavity by a uniform distance.
 15. An extrusion diecomprising a die cavity having a shape corresponding to thecross-sectional shape of the required extrusion, and a preform chamberin communication with the die cavity, the preform chamber being ofsimilar shape to the die cavity but of greater cross-sectional area, sothat regions of the preform chamber communicate with correspondingregions respectively of the die cavity and, in use, extrusion materialpassing through all regions of the die cavity are constrained to move ata substantially uniform velocity, the die cavity including a number ofregions of constant bearing length and each region of the preformchamber which corresponds to one of said regions of constant bearinglength having a bearing length which is related to the dimensions andposition of said region of the preform chamber so that, in use,extrusion material passing through each said region of the preformchamber is constrained to move at a velocity such that the materialsubsequently passes at a uniform velocity through each of saidcorresponding regions of the die cavity which are of constant bearinglength, wherein the width of at least one of said regions of the preformchamber is offset in relation to the width of the corresponding regionof the die cavity.
 16. An extrusion die according to claim 15, whereinall regions of the die cavity are of constant bearing length.
 17. Anextrusion die according to claim 15, wherein said regions of the diecavity which are of constant bearing length are of zero bearing length.18. An extrusion die according to claim 15, wherein a region of thepreform chamber which is of minimum bearing length is of zero bearinglength.
 19. An extrusion die according to claim 15, wherein at leastsome of said regions of the preform chamber each have a width which isthe same predetermined percentage greater than the width of therespective corresponding region of the die cavity.
 20. An extrusion dieaccording to claim 15, wherein at least some of said regions of thepreform chamber each have a width which is greater than the width of therespective corresponding region of the die cavity by the samepredetermined amount.
 21. An extrusion die according to claim 15,wherein the width of at least one of said regions of the preform chamberis symmetrically disposed in relation to the width of the correspondingregion of the die cavity.
 22. An extrusion die according to claim 15,wherein the bearing length of each region of the preform chamber isprovided by a bearing part thereof which is immediately adjacent thecorresponding region of the die cavity.
 23. An extrusion die accordingto claim 15, wherein each region of the preform chamber includes a partwhich is upstream of the bearing part which provides the bearing length,and which upstream part increases in width as it extends away from saidbearing part.
 24. An extrusion die according to claim 23, wherein ashoulder is provided at the junction between said bearing part and saidupstream part of the preform chamber.
 25. An extrusion die according toclaim 15, wherein the die cavity and preform chamber are formed inseparate components which are clamped together with the preform chamberin communication with the die cavity.
 26. An extrusion die according toclaim 15, wherein the die cavity and preform chamber are integrallyformed in a single component.